Phosphoric acid plants are currently operated utilizing a basic and well-known process for the acidulation of phosphate rock which comprises reaction of the rock with sulfuric acid to form phosphoric acid with subsequent reaction of the phosphoric acid, with for example ammonia to produce monoammonium phosphate (MAP) and diammonium phosphate (DAP). The phosphoric acid formed in this process is called wet process phosphoric acid. In this reaction, a by-product is gypsum having the chemical formula CaSO.sub.4.2H.sub.2 O. Essentially, all phosphate rock contains some fluoride, normally in the 3.0 to 4.0% range, and the acidulation reaction usually generates gaseous fluorides.
In recent years, both air and water pollution laws and regulations have become more stringent and are now being enforced more rigorously thus requiring that fluoride evolution from such plants be halted. Further, in recent years, studies have been made on the effects of fluorides contained in the final product and indications seem clear that they may have a deleterious effect on the long range producing ability of the soil when present in fertilizers.
In these systems, any conventional wet process phosphoric acid technology accomplishes two primary objectives, namely: (1) phosphate rock acidulation, and (2) the growth of readily filterable calcium sulfate crystals either as the dihydrate (gypsum), or as the hemihydrate. Conventional phosphoric acid technology carries out both of these objectives essentially simultaneously which leads to a number of environmental and purification problems. The presence of excess strong sulfuric acid in the acidulation phase releases fluorides as HF, SiF.sub.4 and/or H.sub.2 SiF.sub.6. This poses serious fluoride emission and subsequent recovery problems. Furthermore, unless excess sulfate levels are carefully and closely controlled, minute gypsum crystals can and will blind rock particles and usually result in poor P.sub.2 O.sub.5 recovery. The presence of free H.sub.2 SiF.sub.6 in the acid system leads to severe scaling and excessive maintenance costs even with improved design features to minimize this effect.
There is a great deal of art related to attempts to remove the fluoride values from fluorine-containing phosphate rock in the operation of a phosphoric acid plant including methods for suppressing the evolution of fluoride values in the operation of a process and/or attempting to scrub the fluorine from effluent gases and waste water. Two such methods are described in U.S. Pat. Nos. 2,954,275 and 2,976,141 to Carothers et al which use sodium or potassium compounds to suppress the fluorides so that they are concentrated in the gypsum cake. However, these processes were conducted in the presence of sulfuric acid in the acidulation reactor and the process had incomplete control on fluoride decomposition and evolution during acidulation.
Other prior art patents which have attempted to overcome the problem of fluorine evolution and the reduction of the amount of fluorine contained in final products are British Pat. No. 735,086 (1955), which discloses the acidulation of phosphate rock by a two-step procedure using a strong mineral acid such as nitric acid or hydrochloric acid, and U.S. Pat. No. 3,431,096 to Hill et al, which reduces evolution of fluoride values in formation of triple superphosphate fertilizer by reaction of phosphate rock and phosphoric acid wherein ammonia or urea is added to suppress the fluorine evolution. However, in this patent, there is no provision for removal of the fluorine values from the product and therefore even if the fluorine evolution is prevented, the fluorine values will be retained in the resulting product and distributed to the soil when it is used as a fertilizer.
A series of United States patents issuing from the mid-1940's to early 1960's, disclosed processes for the defluorination of phosphate rock and the production of defluorinated calcium phosphates. In these U.S. Pat. Nos. 2,337,498; 2,442,969; 2,893,834 and 2,997,367, the defluorination reaction is carried out by subjecting a mixture of phosphate rock, phosphoric acid and an alkali metal material to calcination, that is, by reaction at temperatures as high as 1000.degree. C. to 2200.degree. C. Obviously, under these conditions, the fluorine would be evolved rapidly, or if not evolved, certainly will remain in the final product, said to be an animal feed.
Two additional patents of pertinence to processes of this type are U.S. Pat. Nos. 2,567,227 and 2,728,635 to Miller which disclose the acidulation of phosphate rock with phosphoric acid to form monocalcium phosphate, cooling to crystallize the monocalcium phosphate and then converting it to dicalcium phosphate by disproportionation. In the earlier patent, it is indicated that the fluorine in the rock is vaporized in the system, circulates throughout the system and/or leaves the system with the calcium phosphate. The latter patent states that the process of U.S. Pat. No. 2,567,227 provided a final calcium phosphate product having a fluorine content too high to be of animal feed grade. The solution to this problem in the later patent was the addition of some dilute sulfuric acid in the acidulation step which would, of course, lead to additional fluorine evolution during the first step.
Patents are also known in the art which acidulate phosphate rock with phosphoric acid and then recover solid monocalcium phosphate by cooling of the resulting solution and recovering the monocalcium phosphate. Processes of this type are disclosed, for example, in U.S. Pat. Nos. 3,494,735 and 3,645,676. In addition, U.S. Pat. Nos. 3,619,136 and 3,792,151 to Case disclose the reaction of phosphate rock with recycle phosphoric acid at temperatures of about 125.degree.-180.degree. F. (52.degree.-83.degree. C.) to form a solution of monocalcium phosphate, reacting the latter solution with sulfuric acid to produce phosphoric acid and hydrated calcium sulfate, separating the hydrated crystals and recycling a portion of the phosphoric acid to the phosphate rock acidulation. These patents point out that under the conditions cited, fluorides are not evolved but remain primarily unreacted and may be found with insoluble materials although a portion remains in the phosphoric acid solution product. Thus, the products would still be contaminated with fluorides. It is also known to react phosphate rock or a solubilized form with sulfuric acid and KHSO.sub.4 in combination with other steps and this reaction is described in U.S. Pat. Nos. 3,697,246 and 3,718,253.
A further pertinent patent is U.S. Pat. No. 4,026,995, issued May 31, 1977 to Case, which teaches the defluorination of monocalcium phosphate/ phosphoric acid solutions by hydrolyzing the soluble calcium fluosilicates to calcium fluoride, phosphoric acid and silicon dioxide. This patent, however, does not contemplate the presence of alkali metal ion in the system, and in fact, states that the prior art has been unsuccessful in this art when alkali metal fluosilicates of low commercial value are precipitated.
Other patents in the phosphoric acid and fluoride art suggest methods for use of scrubbing and recycling plants in an effort to contain or convert the fluorides evolved so that as much as possible of the fluorine can be recovered. Nevertheless, in all of these earlier approaches to the problem, provisions are never made for eliminating or minimizing the substantive amounts of fluoride contained in the final product nor are there provisions made for converting the fluorides to useful products.
It is also known from an article by Pozin et al, Chemical Abstracts, Vol. 77, no. 141900V, 1972, abstracted from Agrochemie 1972, 12 (6), 164-6, and by Ivanov et al, in Journal of Applied Chemistry of the USSR, Vol. 50, no. 6, pp. 1151-3 (1977), that phosphoric acid can be produced from apatite materials such as phosphate rock by decomposition of the rock with phosphoric acid in the presence of soda to liberate fluoride compounds in the form of Na.sub.2 SiF.sub.6. The calcium in the solution is then precipitated by adding sulfuric acid to form gypsum.
In the above-identified previously issued U.S. Pat. Nos. 4,086,322 and 4,160,657, there are disclosed processes by which phosphate rock may be acidulated with phosphoric acid in the presence of potassium ion and silicon dioxide. These processes provide effective procedures for the elimination of fluoride evolution. The latest technology concerning this problem is the above mentioned U.S. Pat. No. 4,160,657, which represents a departure from these prior processes and provides for more economic utilization of potassium fluosilicate in the system wherein both phosphoric acid and potassium ion are regenerated and reused as essential reactants. The present invention provides a still further economical and advantageous process for the commercial area.
It is to be appreciated therefore, that the process of the invention provides an improvement over the processes of the references discussed herein and all other references of which applicants are aware. The present invention provides a system which substantially eliminates the fluoride problem in the acidulation of fluorine-containing phosphate rock with phosphoric acid, enables one to use low grade phosphate rock or matrix, and facilitates the recovery of useful products from the acidulation reaction and the recovery of the fluorine contained in the rock in a usable form. Therefore, the present invention provides a unique combination of steps and advantages not appreciated heretofore in the prior art.